Bluetooth is a wireless communication standard that enables communication between devices using short-wavelength radio waves in the ISM 2.4 GHz band. Data is transmitted in packets between devices that assume a master-slave relationship, possibly with several devices serving in the slave role in a piconet, or ad hoc wireless network of devices using the same physical channel. All of the devices communicating with the master device in the piconet share the master device's clock. The basic clock signal provides clock events, either a rising or falling edge, at 312.5 μs intervals. Two 312.5 μs clock cycles make up one 625 μs time slot. Data is transmitted between Bluetooth devices in the time slots. Data may be sent from the master device to a slave device beginning in even numbered time slots and the master device may receive data from a slave device beginning in the odd numbered time slots.
Bluetooth devices may enter a sleep mode in which some of the electronic circuits are temporarily deactivated or put into low power consumption mode to save battery energy. Bluetooth sleep modes may be of different durations which may be assigned names depending on the duration and other characteristics. In an example, a sniff mode provides that the device sleeps at a reduced power consumption for 50 ms (millisecond), whereas a hold mode allows the device to sleep for 500 ms, and a park mode may permit the device to sleep for 10 seconds. Other durations are also possible.
Bluetooth devices may go into sleep mode several times a second, depending on the use case. Bluetooth devices may be awakened from a sleep mode by expiration of a time duration set by an internal clock or by the device receiving an external signal.